Solar Charge Controllers are required when using solar panels to charge or regulate a battery. Solar charge controllers make sure that the system and all components are protected. The battery/batteries are protected form overcharging as the controller cuts the power when it senses the battery is full.  In turn the battery will be well managed and have a long healthy life. The Solar panels are protected from reverse current and stop them from discharging the battery at night. 
As the chargers become more expensive they can handle larger loads and have added features. Output control to manage lights and other circuits plus more efficient charging. PWM chargers charge up to 30% more efficiently, and MPPT are another 30% more efficient. Sometimes it it worth investing in a more expensive and advanced controller than buying more solar panels. A great option if mounting area is restrictive.